ABSTRACT
The shaping of heated thermoplastic sheet using vacuum and pressure dates back to the 1930s in the United States and a drape-forming process, in which the heated sheet is smoothed by hand over a form, was developed even earlier. Although thermoforming has been improved in speed and sophistication, it is still not considered a major plasticfabricating process. Its main advantages are inexpensive molds and equipment, which make the process feasible for relatively short runs. In addition, there is little limitation on part size, multiple and differing molds may be used, thin-walled (15 mils or less) parts can be formed with reasonable strength, and preprinted sheet can be used, avoiding some secondary operations. The drawbacks of the process, assuming acceptable sheet is available, center around design limitations that restrict the detail in parts (unless expensive tooling is used), limit the sharpness of corners, and require relatively uniform wall thicknesses in finished parts. Trimming of brittle plastics can be a problem, and the appearance of a trimmed edge may not be acceptable. Trimmed 440scrap must be recycled into sheet for best economics, and hot sheet is easily marred when touched by tooling during the thermoforming.
